When it is necessary to supply high voltage (hundreds to thousands of volts) through a dielectric bulkhead or other insulative wall separating air from a volume of oil, for example, it is impractical to run a cable through the wall or through a stuffing box bonded to the wall. Instead high voltage has been commonly supplied by a threaded coupling feeding through the wall and secured to the wall by a nut screwed on the threads of the coupling. Within the insulation of the coupling is a high voltage conductor extending to a terminal on one side of the wall. The mating threads of the coupling and the nut necessarily extend to an opening in the wall which is at or near ground potential with respect to the high voltage on the terminal side. Air tends to become trapped in the space between the nut and the wall and in the mating threads leading to the opening in the wall.
Whether the air space is continuous between the nut and the wall or along the threads, or is broken into one or more air pockets, the air will become ionized by the electric field between the high voltage potential on the oil filled side of the wall and the ground potential at the wall. Although oil will not support a corona the ionized air will support a corona discharge which erodes the dielectric material and sooner or later results in a breakdown in its insulative property and high voltage arcing.
It is the object of the present invention to provide a high voltage bulkhead coupling which eliminates or minimizes air trapped at or near the bulkhead opening and thus greatly reduces failure due to corona and allows supply of substantially higher voltages than prior couplings.